Alkaline electrochemical cells having zinc anodes and manganese dioxide cathodes have achieved commercially success in recent years. Particularly when manufactured in the cylindrical configuration, such cells constitute important sources of portable electrical energy. Alkaline zinc manganese dioxide cells provide substantially more energy vis-a-vis Leclanche cells when used in high current continuous discharge applications.
Historically, alkaline zinc manganese dioxide cells have been used mainly in primary batteries. To date, significant reductions in battery performance after a few recharge cycles delayed the commercialization of secondary alkaline zinc manganese dioxide cells. Several principal problems contributed to this delay.
One problem endemic to such cells arises from the frequency at which they fail due to shorts developing between the metallic anode and the manganese based cathode. Shorts develop because on repeated discharge anodic zinc tends to migrate towards the cathode. Although the electrodes are separated from one another by a barrier-type separator, pathways may develop between the electrodes. For example, if the physical contact between the separator barrier and the bottom of the cell is disrupted, a short can easily develop.
In prior art cells, the seating of the separator barrier within the cell to reduce shorts between the electrodes in alkaline manganese dioxide cells was accomplished in several ways. U.S. Pat. No. 5,108,852 for a Manganese Dioxide Cathode for a Rechargeable Alkaline Cell and Cell Containing the Same, the disclosure of which is hereby incorporated by reference, discloses one such method. In this method, a plastic disc is placed in the bottom of the cell. Then a convolute separator barrier is placed on the disk and a hot melt material is metered to the inside of the separator barrier so that a seal forms only at the interior surface of the separator barrier. An example of a cell constructed in this way is depicted in FIG. 1. Battery Technologies, Inc. of Richmond Hill, Ontario, Canada has manufactured batteries having such a structure.
During repeat cycling of alkaline manganese dry cells, it is critical to ensure that adequate barrier protection is formed and maintained between the anode and cathode of the cell to prevent shorts from occurring between the cathode and the anode. With the design depicted in FIG. 1, it is possible for anodic zinc to migrate through the absorbent and over the top of the plastic disk thereby creating a shorting pathway to the cathode, as depicted in FIG. 2. Additionally, the manufacture of this type of cell involves additional steps, e.g., the disk must be constructed and then placed within the cell and the cell must be spun during metering of the hot melt.
Another method to prevent shorting is taught by Swiss Patent Application No. 669,479. Specifically directed to rechargeable alkaline zinc manganese dioxide cells, this reference utilizes a separator tube as part of a barrier system to prevent an abrupt capacity loss due to shorting experienced in cells after extended cycle life. The Swiss patent teaches three separate approaches, all using a barrier system assembled outside the cell and kept in place in the cell by a hot melt sealant. The first approach involves using a separator tube having only one open end. The closed end of separator tube is affixed to the bottom of the cell with a hot melt sealant. The second approach affixes a separator tube on a plastic disk which is affixed to the bottom of the cell with a hot melt sealant, whereas in the third approach the separator tube is bent inward before being affixed to the plastic disk.
Since the separator tubes disclosed by the Swiss patent all involve completely sealed arrangements, difficulties are encountered in obtaining a sufficient distribution of electrolyte, i.e., the appropriate electrolyte gradient between the electrodes. In order to overcome this problem, several additional manufacturing steps must be taken. Additionally, the insertion of a separator assembly into the cell further reduces the speed in which batteries can be manufactured utilizing the teachings of Swiss Patent Application No. 669,479.
Therefore, an object of the present invention is to provide a rechargeable alkaline manganese dry cell with an insulating barrier at the positive (cathode) end of the cell that completely separates the cathode and anode compartments.
Another object of the present invention is to provide continuous protection against shorting on cycled discharges in rechargeable alkaline manganese dry cells and batteries.
Yet another object of the present invention is to provide rechargeable alkaline manganese dioxide cells and batteries having a simplified overall design.
Still another object of the present invention is to provide rechargeable alkaline manganese dioxide cells and batteries that are easy to manufacture.
Other objects and advantages of the present invention will be apparent from the drawings and the description of the invention.